In the machining of plastic spectacle lenses, a spectacle-lens blank, injection-molded in plastic, is normally present, exhibiting a standardised, finish-machined convex front surface of e.g. a spherical, aspherical or progressive shape. The generally concave back or prescription surfaces are provided by cutting operations with a spherical, aspherical, toroidal, atoroidal, progressive or free-form geometry (progressive surfaces), depending on the desired optical effect. The typical conventional sequence in back-surface processing, after the blocking of the spectacle-lens blank with its front surface on a blocking piece, provides for a milling or turning machining process in order to produce the optically active shape, generally followed by a fine-grinding or polishing process to achieve the necessary surface quality, which, however, may be dispensable in the case of a turning-machined spectacle lens.
Also used for the turning process in the prior art are fast-tool turning machines, in which a turning cutter (lathe tool) can be moved high-dynamically, either with linear reciprocation (see e.g. U.S. Patent No. 7,036,408 or the generic U.S. Patent No. 6,523,443) or with rotation (see e.g. WO-A-99/33611), so that non-rotationally-symmetrical lens surfaces with very good surface qualities can be produced by the turning process.
Following production of the spectacle-lens surface with the desired optical effect, the spectacle lens has to be provided with an identifier, in particular for subsequent processing, specifically the edging of the spectacle lens for matching to the particular spectacle frame. For example, a progressive spectacle lens in accordance with DIN EN ISO 8980-2 must be permanently marked with at least the following information: a) alignment marking; this must comprise at least two markings at a distance of 34 mm, and be located symmetrically relative to a vertical plane through the fitting point or the prism reference point; b) indication of the near-vision magnification in dioptres; and c) indication of the manufacturer or supplier or trade name or trade mark. This standard also recommends, as optional, non-permanent identifiers, further alignment markings, for the distance-vision reference point, for the near-vision reference point, for the fitting point and the prism reference point.
Whereas the permanent identifiers are normally produced by permanent engravings, of which the functional engravings, i.e. the engravings required by the optician for the alignment and assignment of the particular spectacle lens, are generally executed so finely that they cannot be seen with the naked eye in normal light, the non-permanent identifiers are executed by e.g. a temporary lens stamp, which is removed again in the course of the finish-machining of the spectacle lens.
In addition, many spectacle lens manufacturers also offer permanent, individual engravings on the spectacle lens, e.g. engraving of the initials of the spectacle-lens supplier, which is intended to emphasise the mass production of the spectacle lenses and is applied at a location on the spectacle lens where it does not impair vision.
The application of permanent engravings is generally undertaken in an engraving machine separate from the actual machining unit, in which engraving machine a rotatably driven engraving tool with a geometrically determined cutter (milling tool) or a geometrically indeterminate cutter (grinding tool) is guided in a defined machining engagement over the spectacle-lens surface to be marked in order to form the engraving. However, engraving machines are also known in which the engraving is applied to the spectacle lens by laser beam.
In order to avoid the additional use of special diamond tools or high-energy laser radiation in order to apply markings to the spectacle-lens surface, the generic U.S. Patent No. 6,523,443 proposes that these markings be produced directly during the machining process by means of the same tool with which the turning operation takes place, as a result of which all reproducibility problems, which occur on each machine change, are ruled out. For the actual turning operation, this tool must be equipped with a rotary cutter with a defined cutting geometry. With a rotary cutter of this kind, however, only markings comprising fine lines running parallel to the cutting edge can be produced. It would be desirable if in this case, as with the known engraving machines, graphical symbols such as letters, numbers, company logos etc. were reproducible in detail.